This invention relates to a method and apparatus for measuring and controlling the bulk density of granular material. It has particular application to the measurement and control of the bulk density of coal in a coke oven charging system. Bulk density variations in coking coals has been a problem primarily because of variations in the surface moisture of the coal. As surface moisture increases, and the coal is mixed or handled, the bulk density decreases with a corresponding volume increase. As the surface moisture decreases, and the coal is mixed or handled, the bulk density increases with a corresponding volume decrease. It is now generally recognized that one of the most important factors affecting the uniformity of coke oven operations and the quality and quantity of coke produced is the bulk density of coal that is charged into the coke oven. Changes in the bulk density of the coal away from an optimum density not only cause irregularity in coke oven heating and in oven pressure, which are reflected in impaired quality of the coke; but it also causes variations in the oven output adversely affecting the coke yield.
It is known that the addition of a small amount of oil to the granular coking coal will significantly increase its bulk density and that the addition of water will significantly lower its bulk density. Accordingly, various methods and devices have been used with rather poor success to control the bulk density of the coking coal by adding varying amounts of oil and/or water in order to obtain and maintain the coal within an optimum bulk density range which has been determined as the range which will yield the greatest quantity of high quality coke without danger of damage to the brick work of the coke oven.
Heretofore, there has been no satisfactory way in which the bulk density of coal could be gaged on a continuous basis to provide a criterion for the automatic addition of water and/or oil to adjust and maintain the bulk density of the coal within a predetermined density range.
Initially the measurement and control of bulk density of the coal was by resort to batch sampling and manually controlled water and/or oil additions, according to which samples of coal removed from a moving conveyor belt were subjected to a laboratory volume weight bulk density test at a remote point. This method of obtaining the bulk density of coking coal is time consuming and substantial amounts of coal are charged into the oven before any correction in the bulk density can be made by adding oil or water. Further, this method does not provide accurate bulk density determination or control. It is vital to obtain accurate gaging that the coal be subjected to the handling it gets when charged into the oven. A vital part of the present system is to simulate this condition prior to gaging by allowing coal to fall the correct distance or to simulate such fall.
A continuous weight-type test for bulk density has also been suggested. According to the latter proposal, a continuous sample stream of coal is diverted from a main flow onto a fixed speed conveyor used to deliver a constant volume of coal over a weight belt to produce a weight signal which is converted into a bulk density reading. Signals from this system have been instrumented to record bulk density and to automatically control water and oil additions to produce a desired bulk density. This system has had the disadvantage, as have all the sampling techniques, of the bulk density measurement being made on a comparatively small sample, wherein the sample bulk density has often varied from the bulk density of the coal actually charged into the coke oven. Moreover, this system requires additional chutes, hoppers, bins, belts and conveyors which add to the cost and complexity of the required equipment.
It has been proposed in the past to automatically detect the bulk density of coal by directing radiation to the coal as it passes a detecting station, and utilizing the detected radiation to control the addition of oil and water to vary the bulk density of the coal. Macdonald et al U.S. Pat. No. 3,148,971 discloses such a system. This patent does not go into the details of the radiation detector nor the control system, but simply indicates that water and oil rates are increased or decreased in accordance with detected changes in bulk density. In a system to be workable, however, it is not sufficient simply to increase or decrease the rates of water and oil addition. It has been found in the present invention that one of the two fluid components should be varied with respect to the other. Generally the rate of oil addition is considered to be the primary control in the present invention, and the water addition rate is varied not only in accordance with bulk density but also in accordance with the rate of oil addition so as to optimize the control of bulk density. Additionally, it has been found in the present invention desirable to employ digital pulse counting techniques in the detection and control of the bulk density of a granular material. Although digital pulse counting techniques have been employed in the past (see, for example, Willett et al U.S. Pat. No. 3,136,892 directed to the detection of heart rot in power line poles), the present invention utilizes such techniques in bulk density control and in a novel way to ensure control of a process over a predetermined range of counts representing a permissible range of bulk densities over which control is to take place.
It is an object of this invention, therefore, to provide an improved means for accurately, continuously, automatically and economically measuring and maintaining a uniform predetermined bulk density of granular material such as coal charged into a coke oven, which eliminates the above-mentioned disadvantages.